1. Field of the Invention
The present invention relates to an imaging apparatus such as a digital still camera, and more particularly to image data correcting technique relating to chromatic aberration.
2. Related Art
In recent years, a remarkable development has been made in the digital imaging apparatus such as a digital still camera, and a model of a high number of pixels that attains light weight and small size has been rapidly spread. Along with this, a small-sized and thin-shaped lens is also required and developed in an optical system such as a digital still camera.
However, when reduction in size and reduction in thickness progresses in this way, it becomes difficult to adequately maintain various kinds of basic optical performance such as aberration and resolution. Particularly, an influence of the chromatic aberration generated by dispersion characteristics of an optical material of a lens itself poses a large problem in maintaining the optical performance.
The chromatic aberration means, as shown in FIG. 12A, a phenomenon in that image height h on the imaging device 1202 which is a distance from an optical axis 1203 diffuses in a definite range R by dispersion of a white color light 1204 due to a difference in refractive index to a wavelength of light. For an actual subject, it is frequently recognized as a phenomenon that a boundary between dark part and light part of an image on an imaging plane blurs into rainbow color.
FIG. 12B is a view of the imaging device 1202 viewed along a direction of the optical axis 1203. An image height “h” shows the distance from the optical axis 1203 which is a center of the imaging device. Normally, the image height at a position of the optical axis 1203 is defined as 0, and the image height at a position of any one of the vertex of the imaging device 1202 is defined as 1.
Further, as a pixel size of the imaging device becomes smaller as pixel density is increased in the imaging device, the influence of the chromatic aberration per each pixel becomes remarkable. Therefore, correcting the chromatic aberration is a technique indispensable for reducing the size and thickness of the optical system.
As a conventional correcting means of the chromatic aberration, there is an example for storing previously chromatic aberration characteristic data of the optical system and correcting a position of a pixel corresponding to each color of a color filter of the imaging device in accordance with information such as a focal point distance of the optical system. For example, a digital camera disclosed in JP-A-2002-199410 can correct image deterioration in image data caused by the chromatic aberration and a distortion aberration of the optical system, based on chromatic aberration characteristics and distortion aberration characteristics of the optical system stored in a built-in memory.
An output data of the pixel corresponding to each color of the color filter of the imaging device is changed by an ambient light source. Therefore amount of chromatic aberration is not always a fixed value. In daily life, the ambient light source is largely changed from an incandescent bulb to blue sky of clear day, and therefore the chromatic aberration changing amount is also large. Particularly, there is a tendency that the chromatic aberration changing amount becomes large in a range from blue color to ultraviolet. When using a relatively inexpensive small-sized optical system having not so high optical performance, such an optical system suffers largely from the influence of the chromatic aberration.
Therefore, when correcting the image data with only chromatic aberration characteristics of the optical system determined under a certain condition as prior art, changing amount of the chromatic aberration according to the difference of the ambient light source can not be recognized. Thus there is a problem in which deficient or excessive correction of the chromatic aberration may occur depending on the ambient light source on taking an image. Particularly, in a case of the excessive correction, there is a problem of causing an unnecessary deterioration of an image quality such as deterioration of resolution and contrast in peripheral areas of the image.
The present invention is provided to solve the above-described problems, and an object of the invention is to provide an imaging apparatus capable of reducing an image quality deterioration due to the chromatic aberration and its correction, by properly correcting the image data in accordance with changing amount of the chromatic aberration generated from the difference of the ambient light source, even when the relatively inexpensive small-sized optical system having not so high optical performance is used.
Still another object of the present invention is to provide an image data correction method which is effective to processing of image data generated by the imaging device, so-called RAW data, using other information processor such as a personal computer.